When a cat bites a person’s hand and the skin turns red, the wound needs prompt attention to prevent a deep-set infection that’s difficult to treat, a study finds. Nearly one-third of such bites that drove people to seek medical attention required the patient to be hospitalized, and many required surgery, researchers report.

Although cat bites have vexed emergency room doctors for decades, most research has provided only anecdotal information on the risk involved. So Brian Carlsen, a hand surgeon at the Mayo Clinic in Rochester, Minn., and his team reviewed every cat bite to the hand that sent an individual to a Mayo Clinic doctor’s office or ER from 2009 to 2011. Hands are the most common target of cat bites. They also have a lot of tendons and joints, which lack circulation and thus are prone to bad infections because they are largely unprotected by circulating immune cells.

The study included 193 patients, most of whom were treated and released, usually getting oral antibiotics. But 57 patients needed to be hospitalized for an average of three days. Those people received stronger antibiotics, often intravenously, and were monitored in case their conditions worsened, Carlsen says. Indeed, 38 patients didn’t improve on drugs alone and needed surgery to remove dead tissue and clean the wound, the researchers report in the February Journal of Hand Surgery.

Surgery might seem like an extreme response to a cat bite. “But if the tissue is dead,” Carlsen says, “it’s never going to fight the infection. It’s just going to be a culture medium for bacteria.”

Cat saliva typically contains a microbe called Pasteurella multocida, says hand surgeon Jayant Agarwal of the University of Utah School of Medicine in Salt Lake City. “It’s more virulent than staph or strep. It’s a tough customer and needs to be taken seriously.” Pasteurella infections often require penicillin, tetracycline or a broad-spectrum antibiotic, Agarwal says.

The nature of the bites can lead people to underestimate them, says Agarwal: “Cat bites are just little puncture wounds. There’s not a lot of collateral damage. Often patients neglect them and, sure enough, they become infected.”

The new findings provide doctors with a better understanding of the risks associated with these infections. For people who own cats, Agarwal’s advice is straightforward: If redness and swelling appear from a bite, seek medical attention.